Electrical power generation systems powered by variable speed prime movers that require highly regulated electrical output, such as electrical power generation systems used for aeronautical applications, generally use a wound field synchronous machine (WFSM) that serves as an electrical generator. This is because it is easy to adjust rotor current to regulate electrical output of a WFSM. In aeronautical applications, the prime mover is often a gas turbine engine that has a normal rotational velocity that exceeds 20,000 revolutions per minute (rpm). Due to the rotational velocity limitations of the WFSM, such electrical power generation systems generally require a reduction gearbox between the prime mover and the WFSM. This increases weight, cost and complexity of the electrical power generation systems.
Electrical power generation systems may alternatively employ an electrical machine of the permanent magnet type as an electrical generator. Such a permanent magnet machine (PMM) is capable of much higher rotational velocity than a WFSM of similar output and therefore it is capable of direct coupling to the prime mover, thereby eliminating the reduction gearbox. This results in reduced weight, cost and complexity of an electrical power generation system. However, traditional PMMs have no convenient means to alter magnetic flux for regulating their output.
An electrical power generation system may alternatively use a regulated PMM that has a control coil. Current level in the control coil regulates output current of the PMM. A control coil current regulator system senses electrical output potential on a DC bus for the electrical power generation system and adjusts the current level in the control level to regulate the DC bus potential.
This system is very satisfactory except that because the DC bus supplies current to the control coil current regulator system, a separate circuit breaker is necessary to cut off the control coil current regulator system from the DC bus when the DC bus develops a fault such as a short circuit. The need for a separate circuit breaker to isolate the DC bus upon such an occurrence increases weight, size and cost of the electrical power generating system. There is also a need to for a separate circuit breaker to disconnect the DC bus from the faulty load.